1. Field of the Invention
The present invention relates to a process for producing trimellitic acid which can be suitably used as a raw material for plasticizers, polymers having a heat-resisting property, etc., by the oxidation of pseudocumene with a molecular oxygen-containing gas.
2. Description of the Prior Art
Trimellitic acid is a useful compound as a rwa material for thermostable plasticizers or various heat-resistant polymers, so that the development of an effective process by which trimellitic acid having a sufficiently high purity can be prepared at a low cost has been required.
It has previously been known that there are some methods for preparing trimellitic acid by oxidizing pseudocumene with a gas containing molecular oxygen in an acetic acid solvent in the presence of a catalyst comprised of one or more heavy metal compounds and one or more bromine compounds; and the methods characterized by continuously conducting the oxidation are described in for example, Japanese Patent Publication No. 23732/70, Japanese Patent Laid-open Nos. 7173/71, 2932/81, 167942/82, Japanese Patent Publication No. 5898/83, etc..
Although each of these methods has an advantage that the product having a moderately good quality can be continuously obtained, every method is not yet very sufficient in the practical use.
That is to say, the method described in Japanese Patent Laid-open No. 7173/71 is not only a complicated process characterized by conducting the oxidation with multi-stage operations by changing the composition of the quarternary catalyst comprising cobalt, manganese, cerium and bromine from one stage to another, but also an uneconomical process because the recycling of the catalyst which is very expensive is practically impossible by the necessity that the composition of the catalyst has to be adjusted in each stage.
The method described in Japanese Patent Laid-open No. 23732/70 is a multi-stage oxidation process characterized in that the oxidation is successively conducted by introducing cobalt and bromine compounds into the first stage and by adding one or more manganese compounds to the reaction system in the second stage, and the method described in Japanese Patent Laid-open No. 167942/82 is a multi-stage oxidation process characterized in that the reaction in the first stage is conducted by introducing the whole amounts of one or more cobalt compounds and one or more bromine compounds, and a part of one or more manganese compounds until the concentration of the unreacted pseudocumene in the liquid product for the first stage is less than 0.4% by weight, and the oxidation is then completed in the second stage by adding the other part of one or more manganese compounds to the reaction system to re-adjust the composition of the catalyst.
These methods, however, have a disadvantage that the yield of and the selectivity for trimellitic acid are not suffciently high since the oxidation of pseudocumene to carbon dioxide and water, which is the most undesired side reaction for the selective oxidation of pseudocumene to trimellitic acid, is relatively significant because these methods are designed actually for improving the effectiveness of the second stage oxidation by the addition of one or more manganese compounds, and the composition of the catalyst used in the first stage oxidation is, therefore, not sufficiently suitable for the selective oxidation.
The method described in Japanese Patent Laid-open No. 167942/82 has such a risk that the product is contaminated with bromine since a large amount of bromine is used as a catalyst component in the method.
Moreover, these methods have the disadvantages that the operation is complicated and the recycling of the used catalyst is practically difficult because it is necessary to change and limit the composition of the catalyst in each stage.
The method described in Japanese Patent Publication No. 5898/83 is a continuously-flow oxidation process by double-stage operations characterized in that the first stage oxidation is conducted at 110.degree.-170.degree. C. to obtain the first-stage reaction product in which the concentration of pseudocumene is not more than 10% by weight, which is succeeded by second stage oxidation at 180.degree.-240.degree. C. However, this method has a disadvantage that the activity of the catalyst used and the selectivity for trimellitic acid are not practically sufficient; consequently, the yield of trimellitic acid is low because the composition of the catalyst used, in particularly the ratio of bromine to cobalt and manganese, is not kept at a ratio in a suitable range for the desired oxidation, although the problem of the difficulty in recycling the used catalyst can be removed by using the operation that the whole quantity of the catalyst components is charged in the first stage of the reaction.